1. Field of the Invention
The present invention relates to a method and a system of field orientation control for an AC motor. In particular, the present invention relates to a method and a system for field orientation control of an AC motor which uses the impedance variation of an AC motor at a high-frequency region to control the AC motor. The method and the system of field orientation control may be applied to AC motors, which include induction motors.
2. Description of the Related Art
AC motors generally do not have problems related to frequent maintenance and reparation because there is less abrasion in the brush and commutator parts, as compared to DC motors. Thus, AC motors are increasingly applied to various industrial fields that require high performance control systems.
Such AC motors may control the position and speed of rotors only when the position of the field has been determined. Conventionally, sensors detecting position and speed of rotors, such as rotary encoders, have been used to control the position and speed of such rotors.
However, use of such sensors makes AC motor systems more expensive, due to the installation cost of the sensor for detection of the position and speed of the rotors. Also, the control system becomes complicated because the signals from the detection sensor should be converted into signals which may be processed in the control system. Moreover, signals from such detection sensor are susceptible to electromagnetic noise, often causing trouble in AC motor control system with incorrect information from the detection signals when high-frequency signals are injected to the AC motor.
As the above-described problems with the method of using a sensor to detect position and speed of rotors are recognized, sensorless control methods to orient fields without a detection sensor have also been studied.
There are two major types of sensorless control methods. One is to use back electromotive force and the other is to use magnetic variation of impedance. The control method using the back electromotive force shows high efficiency if the speed is medium to high. However, it has low reliability in performance at the low speed range with comparatively high voltage disturbance, because the magnitude of the back electromotive force is proportional to the rotor""s speed. Also, the conventional control methods using the variation of impedance, which inject voltage signals in the middle of sampling cycle or which use rotating high-frequency signals, have demonstrated difficulty in controlling the position and speed of rotors.
In order to solve the above-mentioned problems, the present invention provides a method and a system thereof for sensorless AC motor control, using high-frequency signals.
The present invention also provides a method and a system thereof for AC motor control having high control efficiency regardless of load conditions, by controlling AC motors through current control on the control axis.